1) Field of the Invention
The present invention relates to a thermal fixing unit which is for use in an image forming apparatus including an electrophotographic recording apparatus such as a copying machine, facsimile and laser printer using a toner, and which thermally fixes an image transferred onto a record medium, and more particularly, relates to a method and system for controlling a temperature of the thermal fixing unit, and further to an image forming apparatus based upon that temperature control technique for the thermal fixing unit.
2) Description of the Related Art
In general, in an image forming apparatus (which will be referred hereinafter to as a printer) including an electrophotographic recording apparatus such as a copying machine, a facsimile and a laser printer using a toner, a thermal fixing unit, which thermally fixes a toner image transferred onto a record medium (paper), is located along a record medium conveyance path on the downstream side of a transferring system.
FIG. 16 is an illustration of a common construction of the thermal fixing unit and a temperature control system therefor.
As shown in FIG. 16, a thermal fixing unit, designated at numeral 191, is made up of a heating roller 192 containing a heat source 193 such as a halogen lamp, and a pressure roller (backup roller) 194 for pressing a record medium against the heating roller 192.
In addition, a temperature measuring section 195 and a temperature control section 196 are provided as a temperature control system which takes charge of controlling a surface temperature of the heating roller 192. The temperature measuring section 195 is for measuring the surface temperature of the heating roller 192, and is constructed with a thermo-sensitive device such as a thermister, while the temperature control section 196 is for executing the ON (lights)/OFF (lights-out) -control for the heat source 193.
Secondly, referring to the flow chart (steps S1 to S8) of FIG. 17, a description will be made hereinbelow of a prior control operation in a printer including the aforesaid thermal fixing unit 191, and more specifically, of an operation to be taken for when an operational variation takes place from an ordinary printing operation to a consecutive printing operation or a stand-by mode.
After the completion of a predetermined initializing operation, upon receipt of print data from a host unit such as a host computer, a printer starts to print the print data onto a paper sheet (record medium) (step S1). At this time, as will be described herein later, the surface temperature of the heating roller 192 in the thermal fixing unit 191 is controlled by the temperature control section 196 in accordance with various kinds of printing conditions or requirements, for example, whether or not the printer is in a consecutive printing operation and whether or not being in a starting operation (intermittent printing; the first printing to be done when the printing resumes from the stand-by mode).
After the completion of the printing of the print data from the host unit, in the case of receiving the next print data before a paper discharge sensor (not shown) detects a rear end portion of the paper sheet (YES route from step S2), the temperature control section 196 sets the desired temperature of the heating roller 192 to a target temperature to be controlled for consecutive printing which will be mentioned herein later (step S3), before a printing operation starts (step S1). Incidentally, the aforesaid paper discharge sensor is designed to sense the fact that the paper sheet rear end portion passes through the thermal fixing unit 191.
On the other hand, in the case that the reception of the next print data does not take place before the paper discharge sensor detects the paper sheet rear end portion (NO route from step S2), the temperature control section 196 sets the controlled target temperature for the heating roller 192 to a temperature Tsb for a stand-by mode which will be mentioned herein later (step S4). Incidentally, in the step S4, in order to prevent the excessive heating resulting from the event that the pressure roller 194 comes into contact with the heating roller 192, the controlled target temperature by the heating roller 192 is merely set to the stand-by mode temperature Tsb lower than the controlled target temperature for the regular printing, which does not signify the actual advancement to the stand-by mode.
Following this, a decision is made as to whether the next print data is received or not (step S5), and a further decision is made on whether the time t elapsed after the paper sheet rear end portion passes by the paper discharge sensor of the thermal fixing unit 191 exceeds a predetermined time (for example, 5 seconds) (step S6).
If the reception of the next print data occurs before the time t exceeds 5 seconds (NO route from step S6 and YES route from step S5), the operational flow advances to the step S3 to continue the printing in a consecutive printing mode.
On the other hand, if no transmission of the next print data takes place although the time t exceeds 5 seconds (NO route of step S5 and YES route of step S6), the printer stops in accordance with a given stop sequence, and issues a stop command or the like to stop the rotation of its photosensitive drum (step S7) and then goes into the stand-by mode to wait for the next print data (print instruction) from the host unit (step S8).
Meanwhile, in case where the host unit such as a host computer makes a request for printing to a printer, the print data is evolved in the host unit or in a controller of the printer, and then transferred, together with a print instruction, from the controller to a mechanism control section within the printer. At this time, since the evolution or development time of the print data depends upon printing patterns, the evolution can be shorter or longer than the time required for the preceding printing (preceding print time).
In the case of accomplishing the printing of a plurality of identical patterns, or in the case that a print data evolution time is shorter than the preceding print time as mentioned above, for example as shown in FIG. 18A, the printer implements the consecutive printing while conveying paper sheets at its maximum print speed and at the minimum interval (distance) between the paper sheets. FIG. 18A is an illustration of an example of an output of a paper discharge sensor attainable when the printer conducts the consecutive printing while conveying the paper sheets at the maximum print speed and at the minimum interval.
On the other hand, if the next printing instruction (print data) does not arrive in the printer because of no completion of the evolution of the print data, although a predetermined time (for example, 5 seconds) elapses, as mentioned before with reference to FIG. 17, the printer stops the operations of the rotating system such as the photosensitive drum in accordance with a given stop sequence, and proceeds to the stand-by mode to go into the ready condition until receiving the next printing instruction. Thereafter, when the printing instruction comes, the printing is done in a predetermined printing start sequence. In an extreme example, in the case that a printing instruction comes immediately after a predetermined stand-by time elapses, after once stopping the printing in accordance with a stop sequence, the printer immediately resumes the printing through a restart sequence.
Furthermore, even in the case that the print data evolution time exceeds the preceding print time, for example as shown in FIG. 18B, the printer waits for the arrival of the next printing instruction by a predetermined time (for example, 5 seconds) after the completion of the preceding printing while operating the rotational drive system such as the rollers 192, 194 of the fixing unit 191, thereby surely suppressing the decrease in throughput. FIG. 18B is an illustration of an output of a paper discharge sensor when consecutive printing takes place in a state where the interval between paper sheets (inter-paper interval) varies.
In the case of assuming a ready condition while operating the rotational drive system after the completion of the preceding printing as described above, as mentioned with the step S4 of FIG. 17, for the purpose of avoiding even only a little extra heating of the pressure roller 194, the controlled target temperature for the fixing unit 191 (the controlled target temperature for the heating roller 192) is switched to the stand-by mode temperature Tsb at the time that the paper sheet rear end portion passes by the paper discharge sensor. Since this stand-by mode temperature Tsb is naturally set to be lower than the controlled target temperature for the regular printing operation, the heat source 193 within the heating roller 192 is virtually cut off forcedly by the temperature control section 196. However, because the surface temperature of the heating roller 192 does not immediately drop even if the heat source 193 is cut off forcedly, the pressure roller 194 rotates while coming into contact with the heating roller 192 whose temperature is substantially kept at the controlled target temperature for the regular printing operation.
The controlled target temperature to be given during the consecutive printing is set to be higher than the controlled target temperature for the first printing (at the printing start) to be taken for when the printing resumes in response to the arrival of a printing instruction during the stand-by mode, and for the following reason or situation.
That is, since it takes a long time to warm up the pressure roller 194 at the start of printing, the fixing rate required is securable irrespective of a low temperature of the heating roller 192. Conversely, if the temperature of the heating roller 192 is not set to a relatively low value, an excessive fixing condition is liable to occur to produce wrinkles on paper sheets.
On the other hand, since the pressure roller 194 comes into contact with paper sheets for a longer time but coming into contact with the heating roller 192 for a shorter time during the consecutive printing, the temperature of the pressure roller 194 does not easily rise. Particularly, such a situation occurs noticeably in the case of performing the consecutive printing while conveying the paper sheets at the maximum print speed and at the minimum inter-paper interval (distance) as shown in FIG. 18A. Accordingly, as mentioned above, the controlled target temperature to be taken during the consecutive printing is set to be higher than the controlled target temperature for the start of the printing.
However, when the inter-paper interval is prolonged halfway because of the problems about the data evolution time or the like even during the consecutive printing, since the pressure roller 194 is heated in a state of rotating while coming into contact with the heating roller 192 maintained substantially at the controlled target temperature for the regular printing operation as mentioned before, the temperature of the pressure roller 194 naturally tends to rise.
Meanwhile, in the recent years, as the pressure roller 194, there has been employed a roller made of a sponge (which will be referred hereinafter to as a sponge roller). The sponge roller is easier to warm up as compared with a prior roller made of a rubber, and therefore, the employment of the sponge roller as the pressure roller 194 permits the temperature of the heating roller 192 to be set to a relatively low value. In addition, since the entire fixing unit also becomes easy to warm up, the shortening of the time period needed for the warming-up of the printer at the initial operation becomes feasible.
However, in the case that the aforesaid easy-to-warm sponge roller with a high thermal insulation effect is employed as the pressure roller 194, if the inter-paper interval is prolonged (varies) during the consecutive printing as shown in FIG. 18B, the temperature of the pressure roller 194 has a stronger tendency to rise.
The fixing conditions depend upon the temperature of the pressure roller 194 as well as the temperature of the heating roller 192, and hence, if the sponge roller is used as the pressure roller 194, the fixing is liable to go into an excessive condition. That is, the prolongation of the inter-paper interval destroys the balance between the fixing rate and the occurrence of wrinkles, which is excellent in a short inter-paper interval condition, so that wrinkles appear on the paper sheet after the fixing.
For instance, if the maximum paper sheet conveying speed is set to 83 mm/sec, even in the consecutive printing, the shortest inter-paper interval is at 39 mm while the longest inter-paper interval assumes 39+83.times.5=454 mm, the range therebetween becomes extremely wide. When the inter-paper interval approaches the longest, as mentioned previously, the temperature of the pressure roller 194 reaches a considerable high value. Nevertheless, in the case of the prior temperature control technique described with reference to FIG. 17, since, when print data arrives within 5 seconds after the passage of a paper sheet, the printing is implemented in a manner that the controlled target temperature for the heating roller 192 is set to the controlled target temperature to be taken during the consecutive printing, the next paper sheet is excessively heated by the heating roller 192 and the pressure roller 194, with the result that wrinkles appear on the paper sheet after the fixing.
Particularly, due to the recent progress of OA (Office Automation), there is a greater tendency for the print data evolution time to greatly vary because various types of data mixes on one page. Accordingly, the expectation exists that, even if the print data evolution time thus greatly varies, that is, even if the inter-paper interval within the printer greatly varies, the occurrence of wrinkles on paper sheets after the fixing is certainly preventable to enhance the print quality.
In addition, in recent years, in general, there is an environmental tendency for a plurality of users to make a request, to printers connected to a network, for various kinds of printing processing at an arbitrary time. Accordingly, the expectation in the thermal fixing also exists that, even under such an environment, the maintenance of the print quality and the prevention of the decrease in throughput are achievable to improve the performance of an information processing system.